Does Optimizing the Placement of Machine Isocenter Affect the Overall Optimized Plan Obtained Using Tomotherapy Treatment Planning System ? A Dosimetric and Analysis Study

Purpose:To study the effect of isocenter placement with respect to DICOM center of patient CT images, on IMRT optimization using TomoTherapy Hi-ART(TM)

Methods:The unique feature of TomoTherapy Hi-ART(TM) machine is the presence of flattening-filter-free(FFF) beam, off-axis IMRT optimization capability and rotational delivery. To cover all possibilities, we choose seven different treatment sites (3 Head and Neck(HN), 1 Hippocampal avoidance, 1 SBRT and 2 brain cases) previously planned and treated on Tomotherapy for this study. For fair comparison planning parameters, used previously for clinical patient treatment were used and left unchanged throughout the study for each case. Four plans were generated for each patient in which the center of patient CT datasets were shifted incrementally from +5 cm to -10 cm with respect to isocenter in the AP direction. Treatment time as a function of planning target volume (PTV) distances from isocenter was studied. Homogeneity index (HI), Target coverage (TC) for PTVs and the mean and max doses of organs at risk (OARs) as a function of the respective distances from the isocenter was also studied.

Results:Up to 13% reduction in treatment time (~100 sec) for HN cases was obtained with PTV coverage by changing the distance of PTV from isocenter by 1.5 cm. Particularly, for SBRT, 3% reduction (80 sec) in treatment time was obtained with identical PTV coverage while decreasing max cord dose by 2 Gy.

Conclusion:Optimization of patient position can thus be accomplished in rotational IMRT delivery by decreasing the distance of PTV from the isocenter while maximizing the distance of OARs from the isocenter thus minimizing treatment time and maximizing organ sparing without compromising tumor coverage. This work can be extended for doing fair comparison of plan quality achieved using various emerging technologies based on FFF modes of delivery.